1. Field of the Invention
The present invention relates to a valve timing control system for an internal combustion engine for controlling operation timings of an intake valve and an exhaust valve of an internal combustion engine, and more particularly to detection of failure of a valve timing control system.
2. Description of the Related Art
As a conventional valve timing control system for an internal combustion engine, there is a technique in which an operation state of a cam angle modifying unit is detected, a drive signal from a drive unit in a given operation state is learned, and based on this learned value, the drive signal to be calculated by a control unit is compensated (for example, a technique disclosed in JP 06-159105 A).
The above-mentioned conventional valve timing control system for the internal combustion engine includes: a relative rotational angle detection unit for detecting a relative cam angle from a crank angle sensor and a cam angle sensor; a target relative rotational angle calculating unit for calculating a target cam angle which is optimal depending on the engine operation state; a learning unit for learning an output value outputted from a control unit to a drive unit, which was calculated based on a differential between the target relative rotational angle calculated by the target relative rotational angle calculating unit and the relative cam angle detected by the relative rotational angle detecting unit; the control unit for controlling the drive unit according to information obtained from the relative rotational angle detection unit, the target relative rotational angle calculation unit, and the learning unit; the drive unit for driving a valve timing adjusting unit; and the valve timing adjusting unit for modifying the cam angle with respect to the crank angle.
The learning unit learns the output value outputted from the control unit to the drive unit if the differential between the detected value detected by the relative rotational angle detection unit and the calculated value calculated by the target relative rotational angle calculation unit is within a predetermined range. The control unit calculates a control amount based on the learning value and drives the drive unit such that the target relative rotational angle and the relative rotational angle coincide, and makes the valve timing adjusting unit operate at a predetermined operating speed.
Further, there is a valve timing control system for an internal combustion engine, which executes a failure determination if the differential between the target value and the detected value is greater than a predetermined value continuously for a predetermined time duration (for example, a technique disclosed in JP 2000-64862 A).
The conventional valve timing control systems for an internal combustion engine are constructed as described above. According to those techniques, in a case where the drive unit control amount learning value has not yet been learned, the valve timing adjusting unit does not operate at the predetermined operation speed. As a result, in the case where the failure is to be determined in the case where differential between the target value and the detection value continues for the predetermined time duration, there is a danger of the failure being determined erroneously.